In a typical full duplex data communication system, a local terminal simultaneously transmits data to and receives data from a remote terminal via a communication channel which is formed at least in part by a two-wire telephone line. A hybrid coupler is located at each end of the channel for the purpose of isolating the incoming and outgoing signals. Because the hybrid couplers operate imperfectly, the outgoing signal of the local terminal may be partially reflected at the local hybrid coupler in the form of a near-end echo and partially reflected at the remote hybrid coupler in the form of a far-end echo. Both echoes corrupt the incoming signal of the local terminal.
The near-end echo is generally identical in carrier frequency to the local outgoing signal. It can therefore be eliminated easily by a conventional near-end echo canceller. The near-end echo canceller comprises an adaptive linear transversal filter that simulates the transfer function of the near-end echo path that produced the near-end echo. The outgoing signal is processed by the near-end echo canceller to generate a simulated near-end echo signal. The simulated near-end echo signal is then subtracted from the incoming signal which is corrupted with the near-end echo.
Correcting the far-end echo is not as simple because the far-end echo may have a continually shifting phase, for example, in the form of a frequency offset, relative to the original outgoing signal. This may be caused by small differences between the carrier frequency of the outgoing signal and the carrier frequency of the far-end echo signal that occur when circuits used to step up and step down the signal carrier frequency are not perfectly matched. This is the case when the communications channel between the local data terminal and the remote data terminal includes a satellite hop. Because of the non-linearity in the far-end echo transfer function as represented by the changing phase, a far-end echo canceller in the form of a linear transversal filter is unable to adequately simulate the far-end echo transfer function (see, e.g., Ling, U.S. Pat. No. 4,813,073).
Thus, conventional far-end echo cancelers are quite complex. For example, in the case where Quadrature Amplitude Modulation (QAM) is utilized to modulate the data symbols onto a carrier frequency of .omega..sub.c /2.pi., a conventional far-end echo canceller for simulating a far-end echo path comprises four transversal filters, followed by two summers, followed by two multipliers, followed by another summer. The disadvantage of this far-end echo canceller is that it is very complex.
Accordingly, it is an object of the present invention to provide a far-end echo canceller for use in a system where the far-end echo signal system has a frequency offset, which echo canceller is simpler in construction than the above-described conventional echo canceller. Furthermore, it is an object of the present invention to provide a far-end echo canceller which is of simple construction for a communication system which utilizes QAM.